Radical loss in a chain reaction of CO and NO in the presence of water: Implications for the radical amplifier and atmospheric chemistry

Atmospheric pressure rate coefficients for the loss of HO2, CH3O2, and C2H5O2 radicals to the wall of a ¼ Teflon tube have been measured. In dry air, they are 2.8 ± 0.2 s-1 for HO2 and 0.8 ± 0.1 s-1 for both CH3O2 and C2H5O2 radicals. The rate coefficient for HO2 loss increases markedly with the relative humidity of the air; however, the organic radicals show no such dependence. These data are used in a kinetic model of the radical amplifier chemistry to investigate the reported sensitivity to water concentration. The increased wall loss accounts for only some of the observed water dependence, suggesting there is an unreported water contribution to the gas phase chemistry. Including the reaction of the HO2/water adduct with NO to yield HNO3 or HOONO into the mechanism is shown to provide a better simulation of the observed water dependence of the radical detector. This reaction would also be important in atmospheric chemistry as it provides an additional loss mechanism for both radicals and NOx.